1. Field Of The Invention
The present invention relates to kaolin clay pigments of a type employed to provide smooth, opaque surface finishes on fibrous web substrates such as high quality printing papers and the like. In particular, the present invention is concerned withsuch kaolin clay pigments and coatings for paper and other fibrous web substrates containing the same, which coatings have a high air voids content which contributes to good optical properties of the coated substrate.
2. Description Of The Related Art
Various pigments have been developed in the art which yield high void-containing coating structure. It is believed that such "high bulk" or high void content structures promote efficient light scatter, which is a property quantifiable as a scattering coefficient, and generally provides a measure of the opacifiying power of the pigment. The advantage of high bulking clay pigments has been generally recognized by the prior art and various methods have been proposed for making such high bulking materials. By way of example, Bundy et al U.S. Pat. Nos. 4,075,030, 4,076,548 and 4,078,941 disclose that high bulk kaolinite may be obtained by selective flocculation, using an organic polyamine flocculant and an acid, of an ultrafine particle size kaolinite.
A general discussion of methods of preparing high-bulking clay pigments is set out in a paper by Bundy et al entitled "Chemically Induced Kaolin Floc Structures For Improved Paper Coating" at pages 175-187 of the proceedings of the 1983 Coating Conference of the Technical Association of the Pulp And Paper Industry (TAPPI).
The use of zirconium carbonates as insolubilizers for adhesives and binders used in paper coating applications is known in the art as indicated by U.K. Pat. No. 1,373,634, which is primarily concerned with the preparation of ammonium and potassium zirconium carbonates. The patent indicates that in addition to their use in aqueous dispersions of polymers, such as emulsion paints, the ammonium zirconium carbonate solutions are useful for insolubilizing the starch binders employed in paper coating processes.
U.K. Pat. No. 956,748 discloses that ammonium zirconyl carbonate may be employed to insolubilize a proteinaceous composition; example 2 of this patent discloses a clay coating slip including a proteinaceous adhesive and ammonium zirconyl carbonate.
U.K. Pat. No. 1,001,997 discloses the utilization of ammonium zirconyl carbonate to alter the permanency and water resistance of modified latex coating compositions such as copolymers which may contain inorganic pigments such as clay.
U.K. Pat. No. 1,024,881 discloses the use of ionic zirconium salts, more particularly ammonium zirconyl carbonate, to insolubilize starches employed as binders for coatings on paper and paperboard. In such system, the zirconium compound is added to a binder-containing composition to effect reaction of the zirconium compound with the binder material.
The use of ammonium zirconium carbonate in coating color compositions as an insolubilizer for binders is also disclosed in a publication entitled "Ammonium Zirconium Carbonate, An Alternative Insolubilizer For Coating Binders" at pages 165-179 of the TAPPI proceedings of the 1982 Coating Conference. At page 166 of this report, under the heading "Water Resistance Of Coated Paper," the preparation of coating colors by adding ammonium zirconium carbonate to a slip of clay, natural calcium carbonate and cooked starch or protein is disclosed. Under the heading "Experimental" beginning on page 174 of this paper, the preparation of various pigment slips and coating colors and the effect of ammonium zirconium carbonate on their rheology is described with viscosity test results being shown in FIGS. 9 and 10. Although the aforementioned tests include adding ammonium zirconium carbonate to clay slips in the absence of a binder, their purpose is to attempt to isolate the effect of ammonium zirconium carbonate on clays in the context of its potential use as an insolubilizer for binders or adhesives used in coating colors. For example, the abstract of this TAPPI paper states that the purpose of the paper is to consider ammonium zirconium carbonate as an insolubilizer for coating binders and is not suggestive of any other use of ammonium zirconium carbonate.